space of
time which, geologically speaking, is very short indeed.

Let us then inquire first into the character of the evidence we should
expect to find of such changes of climate, if they have occurred; we shall
then be in a better position to estimate at its proper value the evidence
that actually exists, and, after giving it due weight, to arrive at some
conclusion as to the theory that best explains and harmonises it.

_Effects of Denudation in Destroying the Evidence of Remote Glacial
Epochs._--It may be supposed, that if earlier glacial epochs than the last
did really occur, we ought to meet with some evidence of the fact
corresponding to that which has satisfied us of the extensive recent
glaciation of the northern hemisphere; but Dr. Croll and other writers have
ably argued that no such evidence is likely to be found. It is now
generally admitted that sub-aerial denudation is a much more powerful agent
in lowering and modifying the surface of a country than was formerly
supposed. It has in fact been proved to be so powerful that the difficulty
now felt is, not to account for the denudation which can be proved to have
occurred, but to explain the apparent persistence of superficial features
which ought long ago to have been destroyed.

A proof of the lowering and eating away of the land-surface which every one
can understand, is to be found in the quantity of solid matter carried down
to the sea and to low grounds by rivers. This is capable of pretty accurate
measurement, and it has been carefully measured for several rivers, large
and small, in different parts of the world. The details of these
measurements will be given in a future chapter, and it is only necessary
here to state that the average of them all gives us this result--that one
foot must, on an average, be taken off the entire surface of the land each
3,000 years in order to produce the amount of sediment and matter in
solution which is actually carried into the sea. To give an idea of the
limits of variation in different rivers it may be mentioned that the
Mississippi is one which denudes its valley at a slow rate, taking 6,000
{62} years to remove one foot; while the Po is the most rapid, taking only
729 years to do the same work in its valley. The cause of this difference
is very easy to understand. A large part of the area of the Mississippi
basin consists of the almost rainless prairie and desert regions of the
west, while its sources are i